A little suggestion - you should keep the value of strip_tags($_POST['sector']) in a variable. It helps to not repeat yourself. As for the actual question... are you forgetting the name attribute of the radiobutton? You need to set them all to sector. Also, your form's method needs to be POST...
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minitech♦Nov 8 '11 at 3:24

How are you calling this function? You're appending to $to but we can't see what $to begins as.
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Michael BerkowskiNov 8 '11 at 3:25

You specify in your question that you have tags reading <input type="radio" value="value1">, but did you remember to give those input buttons a name? So they should read <input type="radio" name="sector" value="value1">.
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Matt FNov 8 '11 at 3:26

So instead of $to .= "email.."; use just $to = "email"; so without dot before =

Why?

Because if you use .= it means that you add a value to existing value. So example if $to already contains this value: email@email.com and then you use $to.= "email1@email.com"; then $to will contains BOTH values and will look like this: email@email.comemail1@email.com
Which is not really OK. The other (same) solution is also this: